1. Field of the Invention
This invention relates to nonlinear optical systems, and particularly to organic and organometallic complexes capable of second harmonic generation (SHG) and having other useful nonlinear optical and electro-optic properties.
2. Description of Related Art
Nonlinear second order optical properties, such as second harmonic generation and the linear electrooptic effect, arise from the first nonlinear term, .chi..sup.(2) EE, in the dipolar approximation of the polarization induced in a material by an applied electric field, E.
P(induced)=.chi..sup.(1) E+.chi..sup.(2) EE+.chi..sup.(3) EEE+. . . The vector quantities P and E are related by tensor quantities X.sup.(1), X.sup.(2), X.sup.(3) . . . , where X.sup.(1) is the linear susceptibility, X.sup.(2) is the second order susceptibility which arises from the second order molecular hyperpolarizability, X.sup.(3) is the third order susceptibility which arises from further hyperpolarizabilities, etc. As tensor quantities, the susceptibilities, X.sup.(i), are highly symmetry dependent; odd order coefficients are nonvanishing for all materials but even order coefficients, e.g., .chi..sup.(2) which is responsible for SHG, are nonvanishing only for noncentrosymmetric materials.
Franken, et al., Physical Review Letters, 7, 118-119 (1961), disclose the observation of second harmonic generation (SHG) upon the projection of a pulsed ruby laser beam through crystalline quartz. The use of a laser remains the only practical way to generate an E large enough to be able to detect the SHG phenomenon.
Although a large number of organic and inorganic materials capable of SHG have been found, an intensive search continues for molecules which exhibit large hyperpolarizabilities, .beta.. An organic molecule having a conjugated .pi.-electron system or a low-lying charge transfer excited state often has an extremely large molecular hyperpolarizability, but unfavorable alignment in the crystalline phase can result in a centrosymmetric material in which .chi..sup.(2) vanishes. It is possible to circumvent this problem by using a chiral molecule to insure a rigorously noncentrosymmetric crystal, but problems associated with the creation and maintenance of a high level of optical purity limit the value of this approach. Moreover, optical activity does not guarantee that X.sup.(2) will be large, only that it will be nonzero.
Tomaru, et al., J. Chem. Soc. Chemical Communications, 1207-1208 (1984), disclose SHG in inclusion complexes between dimethyl .beta.-cyclodextrin as a host molecule and a guest molecule chosen from p-nitroaniline, 2-hydroxy-4-nitroaniline and N-methyl-4-nitroaniline. The guests have a crystalline centrosymmetric geometry which is removed by formation of the inclusion complex.
Wang, et al., Chemical Physics Letters, 120, 441-444 (1985), disclose SHG with a crystalline 1:1 inclusion complex between p-nitroaniline and .beta.-cyclodextrin (CD) as a host when exposed to the 1.06 .mu.m output of a Nd-YAG laser. The authors also disclose SHG for CD inclusion complexes of other guests, specifically p-(N,N-dimethylamino)cinnamaldehyde, N-methyl-p-nitroaniline, 2-amino-5-nitropyridine and p-(dimethylamino)benzonitrile.
Cyclic inclusion complexes are those in which the host is a macrocyclic molecule characterized by a relatively large diameter hole in the center. Such compounds are known to include smaller molecules inside the cavity created by the interior void. A much larger class of inclusion complexes is the lattice inclusion complexes, in which the host co-crystallizes with the other material (guest) included within the lattice structure. These complexes are distinct from the cyclic inclusion complexes because for the former the region of the final crystalline structure in which the guest is located is defined by the voids created by the arrangement of host atoms in the unit cell of the host structure.
A useful review of the art relating to nonlinear properties of organic materials is given in Nonlinear Optical Properties of Organic and Polymeric Materials, D. J. Williams, ed., American Chemical Society, Washington, D.C. (1983). The structures, physical properties and applications of known inclusion compounds are reviewed in Inclusion Compounds, Atwood, et al., eds., Academic Press, London (1984). These publications do not disclose utility of lattice inclusion compounds for SHG.
Nesmeyanov, et al., Doklady Chem., 221, 229-231 (1975), translated from Doklady Akademii Nauk SSSR, 221, 624-626 (1975), disclose the separation of metallocenes by inclusion compounds with thiourea. Specifically mentioned are the adducts with ferrocene, nickelocene, and cyclopentadienyltricarbonylmanganese.
Clement, et al., J. Chem. Soc. Chemical Communications, 654-655 (1974), disclose that ferrocene or mixtures of ferrocene and nickelocene form clathrates with thiourea. They report for the ferrocene adduct a molecular ratio of thiourea to ferrocene of 3.0 to 1. Bozak, et al., Chemistry Letters, 75-76 (1975), disclose the incorporation of cyclopentadienylmanganese tricarbonyl (cymantrene) into a ferrocene clathrate of thiourea when the ferrocene to cymantrene weight ratio is about 4:1.
The search continues for other useful material for SHG.